The design and process improvements made in electronic manufacturing technologies such as computer disk drives, microprocessors and memories, and flat panel displays have, and will continue to force orders of magnitude improvement in the process controls of electrostatic discharge (ESD). Clean room material handling systems that support these processes are challenged to simultaneously improve cleanliness and ESD at increasing rates.
Clean room equipment was designed in the early-mid 1980's to operate satisfactorily in Class 1000 and 100 clean rooms, in the early 1990's to Class 10 requirements, and today to Class 1 environments (as defined by FED-STD 209E). Particle size constraints likewise became increasingly stringent, initially from 0.7, then to 0.5 micrometer, more recently to the 0.15 or 0.1 micrometer diameter sizes of today.
Electrostatic charge concerns associated with process equipments were identified by users in the late 1970's and early 80's. Three primary reasons for minimizing static charges are:                1.) Combustible materials or solvent vapors can be ignited by ESD generated arcs from surface potentials of 3000 volts or less.        2.) Dust is attracted by opposite polarity particle charging of a variety of non-conductive products such as plastics, tapes, and certain liquids.        3.) Static sensitive electronic components such as metal-oxide-semiconductor (MOS) integrated circuits are vulnerable to ESD events.        
Initial industry concerns were brought forth by the MOS semiconductor device producers, with early ESD limits of several hundred volts. These process related ESD requirements were met using rather traditional design and conductive material selections.
In the early 1990's, the disk drive industry began to develop requirements for material handling equipments with surface voltages of much less than one hundred volts. This more stringent specification included use of conductive and anti-static plastics, such as conductive PVCs, acetal with carbon black, conductive foams, and anti-static and hygroscopic materials. These materials were generally sufficient to meet most process applications in combined terms of cleanliness and ESD, but qualification criteria were becoming more stringent and qualification processes/testing were taking much longer.
In late 1996, the disk drive industry began requesting conveyor surface voltages of twenty volts and less. An informal disk drive industry survey taken in early 1997 showed the simultaneous requirement developing for increased equipment cleanliness, and an order of magnitude reduction in conveyor surface voltage. The new “target” specification being set forth by disk drive manufacturers was pointing toward cleanliness of Class 1 to 10 at particle sizes less than 0.3 micron diameters; and a maximum ESD surface voltage of 5 volts.
Meeting these challenging equipment requirements meant that attention should be paid to surface and product interaction (particulation), surface to product triboelectric charge compatibility, material compositions and outgassing, surface conductivity and charge decay rates, and the sometimes simultaneous testing of critical parameters. In addition to traditional design and additional material concerns, techniques for low voltage measurements for products on moving conveyor surfaces were designed, tested and verified.
The EOS/ESD Association (Electrical OverStress/ElectroStatic Discharge) has established specific terminology associated with surface resistance:
Insulative:R = 1 E 1011 ohms or higher. (S11.11)Static Dissipative:R = 1 E 104 OHMS TO 1 E 1011 ohms (S11.11)Conductive:R = 0 ohms to 1 E 104 ohms (S11.11)Antistatic:R = n/a. Used to define non-triboelectric materialsHowever, the term “electrically conductive” as used herein includes the range of resistances from zero ohms to about 1×107 ohms.
Producing a low ESD conveyor utilizing slippable rollers is a challenge. Typically, the slippable rollers are fabricated from a synthetic material, chosen for low wear, low outgassing, low particulate creation, low noise, but which is electrically insulative. Some slippable roller conveyors have resistances from an outer diameter of a slippable roller to earth ground of 1012 ohms. Further, the drive pulleys for the roller shafts are chosen for good, long-term wear resistance when driven with a urethane belt. However, the synthetic materials typically chosen for drive pulleys and the urethane belts typically generate thousands of electrostatic volts from a triboelectric effect. Further, typical pulley materials are not electrically conductive.
What is needed is a slippable roller conveyor apparatus that protects the conveyed product from damaging levels of electrostatic discharge. The present invention does this in a novel and unobvious way.